K2-18

Red dwarf star in the constellation Leo

K2-18

Artist's impression of the K2-18 system, with K2-18 on left, K2-18b on right, and K2-18c between.
Credit: ESA/Hubble

Approximate two-dimensional location of the star (in red circle); Sigma Leonis is the nearest bright star, which is in a southerly direction, and the boundary of Virgo is similarly far.

Observation data
Epoch J2000.0 Equinox J2000.0
Constellation | Leo[1]
Right ascension | 11h 30m 14.51774s[2]
Declination | +07° 35′ 18.2553″[2]
Apparent magnitude (V) | 13.50[3]
Characteristics
Evolutionary stage | Red dwarf
Spectral type | M2.8[4]
Astrometry
Radial velocity (Rv)| 0.02±0.52[2] km/s
Proper motion (μ)|  RA: −80.479 mas/yr[2]
Dec.: −133.007 mas/yr[2]
Parallax (π)| 26.2469±0.0266 mas[2]
Distance| 124.3 ± 0.1 ly
(38.10 ± 0.04 pc)
Details
Mass| 0.495±0.004[5] M☉
Radius| 0.469±0.010[5] R☉
Luminosity| 0.0234[6] L☉
Temperature| 3,645±52[7] K
Metallicity [Fe/H]| +0.10±0.12[7] dex
Rotation| 38.9 to 40.2[7] days
Age| 2.9 to 3.1[7] Gyr
Other designations
K2-18, EPIC 201912552, TIC 388804061, 2MASS J11301450+0735180[8]
Database references
SIMBAD| data
Exoplanet Archive| data

K2-18, also known as EPIC 201912552, is a red dwarf star with two planetary companions located 124 light-years (38 parsecs)[5] from Earth, in the constellation of Leo.

Its name is because it was discovered by the K2 Mission, which extended the mission of the Kepler Space Telescope after failure of two of its reaction wheels.

## Planetary system

[edit]

See also: K2-18b

The star has a transiting exoplanet, called K2-18b, a super-Earth located within the habitable zone of K2-18.[9][10] It was discovered in 2015 by the Kepler space telescope in its K2 mission.[4] It is the first exoplanet in the habitable zone, albeit a hydrogen-rich sub-Neptune,[11] to have its atmosphere characterized; initially thought to contain water vapor,[12] more recent observations have instead detected methane and carbon dioxide.[13] The presence of these molecules and non-detection of ammonia is consistent with predictions for a hycean planet.[13]

A second, non-transiting planet, K2-18c, was discovered in 2017 by radial velocity with HARPS.[14] This planet was challenged by another team with CARMENES data,[15] but its existence was reaffirmed by the discovery team based on both HARPS and CARMENES data.[5] This planet has also been confirmed by a later independent study.[16] System tidal simulation suggests that K2-18c is a gas-rich, Neptune-like planet, similar to K2-18b.[17]

The K2-18 planetary system[17] Companion
(in order from star) | Mass | Semimajor axis
(AU) | Orbital period
(days) | Eccentricity | Inclination | Radius

|
|
|
|
|
|

c | ≥5.62±0.84 M🜨 | 0.0670 ± 0.0002  | 9.2072±0.0065[16] | <0.2  | —  | —
b | 8.63±1.35 M🜨 | 0.1591±0.0004  | 32.94488±0.00281  | —  | —  | 2.711±0.065 R🜨

## References

[edit]

1. ^ Roman, Nancy G. (1987). "Identification of a constellation from a position". Publications of the Astronomical Society of the Pacific. 99 (617): 695. Bibcode:1987PASP...99..695R. doi:10.1086/132034. Constellation record for this object at VizieR.
2. ^ a b c d e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
3. ^ Zacharias, N.; Finch, C. T.; Girard, T. M.; Henden, A.; Bartlett, J. L.; Monet, D. G.; Zacharias, M. I. (2012). "VizieR Online Data Catalog: UCAC4 Catalogue (Zacharias+, 2012)". VizieR On-line Data Catalog. Bibcode:2012yCat.1322....0Z.
4. ^ a b Montet, Benjamin T.; et al. (5 August 2015). "Stellar and Planetary Properties of K2 Campaign 1 Candidates and Validation of 17 Planets, Including a Planet Receiving Earth-like Insolation". The Astrophysical Journal. 809 (1) 25: 25. arXiv:1503.07866. Bibcode:2015ApJ...809...25M. doi:10.1088/0004-637X/809/1/25. S2CID 33348734.
5. ^ a b c d Cloutier, Ryan; et al. (7 January 2019). "Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES". Astronomy & Astrophysics. 621 A49: A49. arXiv:1810.04731. Bibcode:2019A&A...621A..49C. doi:10.1051/0004-6361/201833995. S2CID 118828975.
6. ^ Martinez, Arturo O.; et al. (2017). "Stellar and Planetary Parameters for K2's Late-type Dwarf Systems from C1 to C5". The Astrophysical Journal. 837 (1) 72: 72. arXiv:1701.00588. Bibcode:2017ApJ...837...72M. doi:10.3847/1538-4357/aa56c7. S2CID 25502982.
7. ^ a b c d Sairam, Lalitha; Madhusudhan, Nikku (2025-03-25). "Characterising M dwarf host stars of two candidate Hycean worlds". Monthly Notices of the Royal Astronomical Society. 539 (2): 1299. arXiv:2503.19908. Bibcode:2025MNRAS.539.1299S. doi:10.1093/mnras/staf528.
8. ^ "K2-18". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2024-04-13.
9. ^ "HABITABLE EXOPLANETS CATALOG". UPR. 7 March 2016. Retrieved 12 September 2019.
10. ^ "EPIC 201912552 b reality check drewexmachina 11-22-2015". Drew ExMachina. 7 March 2016. Retrieved 12 September 2019.
11. ^ No, the Exoplanet K2-18b Is Not Habitable
12. ^ Benneke, Björn; Wong, Ian; Piaulet, Caroline; Knutson, Heather A.; Lothringer, Joshua; Morley, Caroline V.; Crossfield, Ian J. M.; Gao, Peter; Greene, Thomas P.; Dressing, Courtney; Dragomir, Diana; Howard, Andrew W.; McCullough, Peter R.; Kempton, Eliza M.-R.; Fortney, Jonathan J.; Fraine, Jonathan (December 2019). "Water Vapor and Clouds on the Habitable-zone Sub-Neptune Exoplanet K2-18b". The Astrophysical Journal Letters. 887 (1): L14. arXiv:1909.04642. Bibcode:2019ApJ...887L..14B. doi:10.3847/2041-8213/ab59dc. ISSN 2041-8205. S2CID 209324670.
13. ^ a b Madhusudhan, Nikku; Sarkar, Subhajit; Constantinou, Savvas; Holmberg, Måns; Piette, Anjali A. A.; Moses, Julianne I. (1 October 2023). "Carbon-bearing Molecules in a Possible Hycean Atmosphere". The Astrophysical Journal Letters. 956 (1): L13. arXiv:2309.05566. Bibcode:2023ApJ...956L..13M. doi:10.3847/2041-8213/acf577.
14. ^ Cloutier, R.; Astudillo-Defru, N.; et al. (December 2017). "Characterization of the K2-18 multi-planetary system with HARPS. A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit". Astronomy & Astrophysics. 608: A35. arXiv:1707.04292. Bibcode:2017A&A...608A..35C. doi:10.1051/0004-6361/201731558.
15. ^ Sarkis, Paula; Henning, Thomas; et al. (June 2018). "The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18". The Astronomical Journal. 155 (6): 257. arXiv:1805.00830. Bibcode:2018AJ....155..257S. doi:10.3847/1538-3881/aac108.
16. ^ a b Radica, Michael; Artigau, Étienne; et al. (December 2022). "Revisiting radial velocity measurements of the K2-18 system with the line-by-line framework". Monthly Notices of the Royal Astronomical Society. 517 (4): 5050–5062\. arXiv:2210.08078. Bibcode:2022MNRAS.517.5050R. doi:10.1093/mnras/stac3024.
17. ^ a b Ferraz-Mello, S.; Gomes, G. O. (2020). "Tidal evolution of exoplanetary systems hosting potentially habitable exoplanets. The cases of LHS-1140 b-c and K2-18 b-c". Monthly Notices of the Royal Astronomical Society. 494 (4): 5082–5090\. arXiv:2005.10318. Bibcode:2020MNRAS.494.5082G. doi:10.1093/mnras/staa1110. S2CID 218763252.

## External links

[edit]

* NASA says distant planet could hold life after they spot signs of rare water ocean - MSN News

* v
* t
* e

Constellation of Leo

* List of stars in Leo
* Leo in Chinese astronomy

Stars|

| Bayer|

* α (Regulus)
* β (Denebola)
* γ (Algieba)
* δ (Zosma)
* ε (Algenubi)
* ζ (Adhafera)
* η
* θ (Chertan)
* ι
* κ (Al Minliar al Asad)
* λ (Alterf)
* μ (Rasalas)
* ν
* ξ
* ο (Subra)
* π
* ρ (Shaomin)
* σ
* τ
* υ
* φ
* χ
* ψ
* ω

|

Flamsteed|

* 3
* 6 (h)
* 7
* 8
* 9
* 10
* 11
* 12
* 13
* 15 (f)
* 18
* 19
* 20
* 21
* 22 (g)
* 23
* 26
* 31 (Yunü)
* 33
* 34
* 35
* 37
* 39
* 40
* 42
* 43
* 44
* 45
* 46
* 48
* 49
* 50
* 51 (m)
* 52 (k)
* 53 (l)
* 54
* 55
* 56
* 57
* 58 (d)
* 59 (c)
* 60 (b)
* 61 (p2)
* 62 (p3)
* 64
* 65 (p4)
* 66
* 67
* 69 (p5)
* 71
* 72
* 73 (n)
* 75
* 76
* 79
* 80
* 81
* 82
* 83
* 85
* 86
* 87 (e)
* 88
* 89
* 90
* 92
* 93
* 95 (o)
* 6 LMi
* 18 LMi
* 49 LMi
* 52 LMi

Variable|

* R
* AD
* CN
* CW
* DP
* EE
* GQ

HR|

* 4253 (p1)
* 4459 (Formosa)

HD|

* 81040
* 88133
* 89307
* 96063 (Dingolay)
* 97658
* 99109 (Shama)
* 100777 (Sagarmatha)
* 102272

Other|

* BD+20 2457
* Gliese 408
* Gliese 436 (Noquisi)
* GJ 3685
* HVS 7
* K2-3
* K2-18
* 2MASSI J0937347+293142
* 2MASS J10475385+2124234
* MACS J1149 Lensed Star 1
* PSR B0943+10
* PSR B0950+08
* SDSS J102915+172927
* TOI-6894

| Exoplanets|

* γ1 Leonis b
* Gliese 436 b (Awohali)
* HD 81040 b
* HD 88133 b
* HD 89307 b
* HD 89345 b
* HD 99109 b (Perwana)
* HD 100777 b (Laligurans)
* K2-18b

|

Galaxies|

| Messier|

* 65
* 66
* 95
* 96
* 105

|

NGC|

* 2903
* 2906
* 2911
* 2964
* 3041
* 3048
* 3049
* 3177
* 3185
* 3187
* 3190
* 3193
* 3226
* 3227
* 3239
* 3274
* 3301
* 3338
* 3357
* 3362
* 3367
* 3370
* 3377
* 3384
* 3412
* 3426
* 3447
* 3455
* 3489
* 3501
* 3506
* 3507
* 3521
* 3524
* 3535
* 3593
* 3596
* 3598
* 3599
* 3602
* 3605
* 3607
* 3608
* 3628
* 3632
* 3640
* 3646
* 3647
* 3664
* 3666
* 3678
* 3681
* 3684
* 3686
* 3697
* 3705
* 3710
* 3758
* 3799
* 3800
* 3801
* 3805
* 3806
* 3810
* 3816
* 3821
* 3837
* 3840
* 3841
* 3842
* 3844
* 3845
* 3851
* 3857
* 3859
* 3860
* 3861
* 3862
* 3864
* 3867
* 3868
* 3873
* 3875
* 3883
* 3884
* 3886
* 3900
* 3902
* 3908
* 3925
* 3937
* 3947
* 3951
* 3954
* 4002
* 4003
* 4005

Numbered|

* I
* II
* III
* IV
* V

Other|

* CLASS B1152+199
* Cosmic Horseshoe
* FBQ 0951+2635
* IC 2498
* IC 2628
* IRAS 10565+2448
* J1000+1242
* Leo P
* Leo T
* PSO J172.3556+18.7734
* SDSS J1029+2623
* UGC 6614
* UGC 6697

| Galaxy clusters|

* MACS J1149.5+2223

|

| Astronomical events|

* SN 1988Z

|

Category
*[RA:]: Right Ascension
*[Dec.:]: Declination
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